070 telephone numbers are forwarding numbers called "Personal Numbering" They were designed to allow calls to follow the holder during the day. For instance, evenings and mornings they could go to the holder's home number then during work hours to a mobile or work number. They have a totally valid reason for use by Sellers as a means of protecting a private numbers from being broadcast in Newspaper adverts or on "For Sale" signs. They also made sure that a prospective purchaser could contact the seller at any time of day. They are frequently perfectly reasonably allocated to users of sales sites e.g. Private Property Sale sites, during the "for sale" period only. In addition to the other benefits the number would then be re-allocated on sale meaning the holder would no longer receive calls regarding a sold property.
They are very useful then, but what about the frauds and how they fit in? Very simply they confer the magic scam ingredients of hidden identity and untraceability. In other words ANONYMITY. The major use by alleged buyers is to disguise the COUNTRY of the caller as other than the UK. Although the number itself is a UK number the divert can go almost anywhere in the world including Nigeria. This is a facility very much used scammers attempting the Overpayment or Advance Payment Scam although not restricted to this fraud.
The deception is helped by the fact that most people just assume it to be a UK mobile number because it starts with 07 and don't question it. If you are a seller approached by prospective purchaser giving an 070 number it is prudent to establish a fixed landline number for the purchaser before doing any serious business. If the buyer continues to insist this is the only method of contact, make up your own mind whether you really want to do business with someone you may not be able to contact after the event. (House sellers, for instance, often drop the Personal Number once the sale looks serious. There's no need for it anymore.) All of this means that any buyer with an 070 number should be regarded as of questionable veracity and the seller should be on the lookout for other danger signals .
If you are a buyer then you should also act with caution if dealing with an 070 number. There have been cases reported recently of fake Flat or House to let adverts using 070 numbers. This is one case of Fraudulent Deposit Demands where the purpose again is to distance the fraudster from the actual location.